This invention pertains generally to systems and devices for polishing and planarizing substrates, and more particularly to a chemical mechanical planarization or polishing (CMP) apparatus having edge, center and annular zone control of material removal.
Chemical Mechanical Planarization or Polishing, commonly referred to as CMP, is a method of planarizing and polishing semiconductor and other types of substrates. Planarizing a surface of a semiconductor substrate or wafer between certain processing steps allows more circuit layers to be built vertically onto a device. As feature size decreases, density increases, and the size of the semiconductor wafer increase, CMP process requirements become more stringent. Substrate to substrate process uniformity as well as uniformity of planarization across the surface of a substrate are important issues from the standpoint of producing semiconductor products at a low cost. Thus, as the size of structures or features on the semiconductor substrate surface have been reduced to smaller and smaller sizes, now typically less than 0.2 microns, the problems associated with non-uniform planarization have increased.
Many reasons are known in the art to contribute to uniformity problems. These include the manner in which pressure is applied to the backside of the substrate during planarization, edge effect or non-uniformities near the edge of the substrate arising from the typically different interaction between the polishing pad at the edge of the substrate as compared to at the central region, and non-uniform deposition of metal and/or oxide layers that might desirably be compensated for by planarizing or adjusting the material removal profile during polishing. Efforts to simultaneously solve these problems have not heretofore been completely successful.
With respect to the nature of the substrate backside polishing pressure, conventional machines typically use hard backed polishing heads to press the substrate against a polishing surface. That is, the polishing heads have a hard receiving surface that presses directly against the backside of the semiconductor substrate. As a result any variation in the receiving surface of the head, or the presence of any material trapped between the substrate and the receiving surface results in a non-uniform application of pressure to the backside of the substrate. Thus, the front surface of the substrate typically does not conform to the polishing surface resulting in planarization non-uniformities. Moreover, such hard backed head designs often must utilize a relatively high polishing pressure (for example, pressures in the range of between about 6 psi and about 8 psi) to provide any reasonable degree of conformity between the substrate and the polishing surface. However, such relatively high pressures can deform the substrate causing too much material to be removed from some areas of the substrate and too little material from others.
Attempts have been made to remedy the above problems with hard backed polishing heads by providing a soft insert between the receiving surface and the substrate to be polished in an attempt to provide some flexibility in an otherwise hard backed system. This insert is commonly referred to as a wafer insert or more simply an insert. The use of inserts is problematic because they frequently result in process variation leading to substrate-to-substrate variation. This variation is not constant or generally deterministic. One element of the variation is the absorption of water or other fluids such as slurry used in the polishing process. Because the amount of water absorbed by the insert tends to increase over its lifetime, there is frequently process variation from substrate-to-substrate. These process variations may be controlled to a limited extent by preconditioning the insert by soaking the insert in water prior to use and by replacing the insert before its characteristics change beyond acceptable limits. This tends to make the initial period of use more like the later period of use, however, this can increase equipment maintenance costs and decrease process throughput. Moreover, unacceptable process variations are still observed due to, for example, variations in the thickness of the insert, wrinkling of the insert and material being trapped between the hard backed head and the insert or the insert and the substrate.
Moreover, use of inserts also requires a fine control of the entire surface to which the insert is adhered as any non-uniformity, imperfection, or deviation from planarity or parallelism of the head surface would typically be manifested as planarization variations across the substrate surface. For example, in conventional heads, an aluminum or ceramic plate is fabricated, then lapped and polished before installation in the head. Such a complex manufacturing process increases the costs of the head and of the machine, particularly if multiple heads are provided.
To overcome the above problems with hard backed polishing head and polishing heads, some attempts have been made in recent years to utilize soft backed heads, however, they have not been entirely satisfactory. One type of soft backed head is described in U.S. Pat. No. 6,019,671, to Shendon, hereby incorporated by reference. Referring to FIG. 1, a prior art soft backed polishing head 10 typically includes a carrier 12 having a subcarrier 14 with a lower surface 16 on which the substrate 18 is held during the polishing operation, and a retaining ring 20 circumferentially disposed about a portion of the subcarrier. The subcarrier 14 and the retaining ring 20, via a backing ring 22, are suspended from the carrier 12 by a flexible gasket 24 so that they can move vertically and are able to float on the polishing surface (not shown) during the polishing operation. Small mechanical tolerances are provided between the subcarrier 14 and the retaining ring 20 and adjacent elements to accommodate minor angular variations during the polishing operation with little friction and no binding. During the polishing operation a pressurized fluid is admitted into chambers 26, 28, formed by the flexible gasket 24 and the carrier 12 to force the subcarrier 14 and the retaining ring 20 against a polishing surface (not shown). A flexible member 30 or membrane stretched across the lower surface 16 of the subcarrier 14 forms a lower chamber 32 or cavity which is pressurized via a passageway 34 to further press the substrate 18 against the polishing surface.
A primary advantage of a soft backed polishing head 10 lies in the fact that the soft material of the flexible member does not distort the substrate as it is pressed against the polishing pad. As a result, conformity of the substrate front surface to the polishing pad can be achieved at lower polishing pressures and without distortion, providing both improved polishing uniformity and planarization.
While a significant improvement over hard backed heads with or without inserts, prior art soft backed polishing heads are not wholly satisfactory for a number of reasons. One problem with this approach is that it does nothing to reduce or eliminate the non-uniformities due to material trapped between the flexible member and the substrate. Moreover, the use of the flexible member can actually increase non-uniformities by introducing new variables, such as variation in the thickness or flexibility of the flexible member across its surface and possible wrinkling of an improperly installed flexible member.
Another problem with prior art soft backed polishing head is a reduction in polishing performance due to interference by the flexible member with other components of the polishing head. For example, as shown in FIG. 1, during the polishing operation a side or skirt portion 36 of the flexible member 30 can deform or bow out due to the pressure applied to the lower chamber 32. This deformation can reduce or eliminate altogether the small mechanical tolerances provided between the subcarrier 14 and the retaining ring 20, causing friction and binding during the polishing operation. As a result, the polishing head becomes unable to accommodate minor angular variations during the polishing operation, resulting in non-uniformity and poor planarization.
With respect to the desirability of being able to adjust the material removal profile to allow for incoming substrate non-uniform depositions, few if any attempts have been made to provide a machine that affords such compensation. Non-uniform depositions can arise from the structure of circuits formed on the substrate or from characteristics of the deposited layers. For example, copper layers, which have become increasingly common in high-speed integrated circuits, tend to form a convex layer thicker at the center of the substrate than the edge. Thus, it would be desirable to have a polishing apparatus that provided a higher removal rate near the center of the substrate than at the edge.
Therefore, there remains a need for an apparatus that provides excellent planarization, controls edge planarization effects, and permits adjustment the substrate material removal profile to compensate for non-uniform deposition of layers on the substrate.
The present invention relates to a CMP apparatus and method for polishing and planarizing substrates that achieves a high-planarization uniformity across the surface of the substrate.
According to one aspect of the present invention, a polishing head is provided for positioning a substrate on a polishing surface of a polishing apparatus for processing the substrate to remove material therefrom. The polishing head includes a subcarrier adapted to hold the substrate during a polishing operation, the subcarrier having a lower surface, a flexible membrane or member secured to the subcarrier and extending across the lower surface, the flexible member having a receiving surface adapted to engage the substrate to press the substrate against the polishing surface, and at least one control-insert disposed between the flexible member and the lower surface. During the polishing operation a pressurized fluid is admitted to a chamber between the flexible member and the lower surface to force the substrate against the polishing surface. The control-insert can be attached to either the lower surface of the subcarrier, or to an inner surface of the flexible member. The control-insert is adapted to inhibit non-planar polishing of the substrate surface, by providing a variable removal rate across the substrate surface. The control-insert accomplishes this by providing mechanical force or pressure at various locations across the substrate in addition to that provided by the pressurized fluid.
In one embodiment, the control-insert includes an annular ring. The annular ring can be located near an outer circumferential edge of the flexible member to control a removal rate near an outer circumferential edge of the substrate surface. Alternatively, the annular ring may be between an outer circumferential edge of the flexible member and a center of the flexible member to control the removal rate near an annular middle portion of the substrate surface between an outer circumferential edge of the substrate surface and a center of the substrate surface. It will be appreciated that the control-insert can include multiple annular rings, or a disk and at least one annular ring.
In another embodiment, the control-insert includes a disk near a center of the flexible member to control the removal rate near a center of the substrate surface.
In yet another embodiment, the rate of removal of material across the substrate surface can be further controlled by varying a cross-sectional thickness of the control-insert. In one version of this embodiment, the control-insert has a cross-sectional area with a constant thickness. In other versions, the control-insert can have a thickness that continuously, in a linear or non-linear manner, increases or decreases from a point proximal to a center of the flexible member to an outer circumferential edge of the control-insert. In yet another version of this embodiment, the control-insert can have a thickness that first increases in a radial direction in a first region from a point proximal to a center of the flexible member, and then decreases in a second region from the first region to an outer circumferential edge of the control-insert. Alternatively, the thickness of the control-insert can decrease in the first region, and increases in the second region. It will be appreciated that the above variations in cross-sectional thickness can be used with both annular ring and disk shaped control-inserts.
The control-insert can be made of either a metal or a polymeric material. In one embodiment, the control-insert is made of substantially the same polymeric material as the flexible member, and is integrally formed with the flexible member. Alternatively, both the subcarrier and the control-insert are made of metal, the control-insert is integrally formed with the subcarrier.
In still another embodiment, the subcarrier further includes a passageway in communication with the lower surface for providing a pressurized fluid to the chamber, and the flexible member has a thickness having a number of holes extending therethrough to the receiving surface for applying the pressurized fluid directly to the substrate. In one version of this embodiment, in which the control-insert includes an annular ring, it is located in a position relative to the holes to enable the pressurized fluid to be applied directly to the substrate. In an alternative version in which the control-insert is a disk it also has a number of holes positioned relative to those in the flexible member to enable the pressurized fluid to be applied directly to the substrate.
In another aspect the present invention is directed to a polishing head having a support assembly disposed between the flexible member and the lower surface of the subcarrier, the support assembly adapted to attach the flexible member to the lower surface of the subcarrier and to hold the flexible member spaced apart therefrom. In one embodiment, the flexible member has a receiving surface portion with a receiving surface adapted to engage the substrate to press the substrate against the polishing surface during a polishing operation, and a skirt portion disposed circumferentially about the support assembly. Generally, the skirt portion includes a material having a hardness different from that of the receiving surface portion.
Desirably, the skirt portion includes a hardness greater than that of the receiving surface portion. This is desirable where the polishing head, further includes a carrier and a retaining ring, the carrier adapted to carry the subcarrier, the retaining ring circumferentially disposed about the subcarrier. In this embodiment, the skirt portion should be sufficiently hard to prevent the skirt portion of the flexible member from deforming during the polishing operation and touching the retaining ring. Preferably, the skirt portion has a hardness at least about 50% higher than the receiving surface portion. More preferably, the where receiving surface portion has a Durometer of from about 30A to about 60A, and the skirt portion has a Durometer of from about 60A to about 90A. Most preferably, where the receiving surface portion has a hardness with a Durometer of less than about 50A, the skirt portion has a hardness with a Durometer of at least about 70A.